Human neutrophil heterogeneity has been suggested on the basis of differences in chemotactic responsiveness, on the basis of results in our laboratory showing differences in Fc receptor mediated rosetting of IgG coated red cells, and more recently with monoclonal antibodies. Heterogeneity of binding of monoclonal antibodies to human neutrophils was examined using the fluorescent activated cell sorter. We developed an IgG monoclonal antibody, 31D8, that binds strongly to neutrophils that are activated with fMLP and more responsive chemotactically, representing about 95% of normal neutrophils. 31D8 antigen appears at the myelocyte stage of maturation. We used 31D8 to study patients with chronic myelogenous leukemia (CML), a clonal disease and showed that failure to express 31D8 on any mature appearing neutrophils correlated with progression of CML to blast crisis. In other studies we demonstrated that neonate cord blood neutrophils have a decreased number of 31D8 positive neutrophils which correlates with an increase in band forms. Similarly there is a marked decrease in the number of 31D8 cells after severe blunt trauma or after experimental infusion of endotoxin. Band forms and 31D8 weakly reactive cells comprise an overlapping but not identical population. 31D8 expression may be a measure of neutrophil maturation independent of nuclear morphology and may be useful to measure the degree of marrow release of neutrophils. Dr. Segal developed IgG monoclonal antibody, C10, which we have characterized as binding strongly to about 30 to 80% of neutrophils. Percent C10 positive neutrophils in the same individual is constant. Further studies are in progress.